Magnetic resonance imaging (MRI) requires a stable magnetic field. Such fields are provided by iron core resistive electromagnets, superconducting magnets and permanent magnets. In various urban settings fluctuating magnetic fields produced by electric powered subways or other sources can interfere with the imaging process. The fluctuations can act as a source of a varying (unstable) magnetic field during imaging.
One of the approaches which has been taken in the past involves magnetically shielding the MRI system from the external field by using a magnetic shielding material. A similar approach uses a shorted superconducting coil. While these can be very effective they tend to be costly and sometimes difficult to install.
Another approach has been to use a magnetic field sensor to detect the external field and use its signal to control current flowing through loops of wire configured to product a magnetic field which approximately bucks or cancels the external magnetic field. This approach can be practical but has some limitations. 1) It can require large amounts of power to produce the bucking magnetic field. 2) Its magnetic field couples to the imaging magnet which complicates the circuitry necessary to condition, filter and amplify the sensors output as required to control the current in the bucking coils. 3) The bucking coils can be impractical to install in such a way as to produce a truly bucking field.